# HG changeset patch
# User František Kučera <franta-hg@frantovo.cz>
# Date 1543959216 -3600
# Node ID d342de2e09a48d0757aad1baf17b6028094389dd
# Parent  029dc972f1f2dd93c91dc994d2b9aef1a70e3dd1
examples: fstab formatting using -in-fstab, -out-nullbyte, xargs and Perl

diff -r 029dc972f1f2 -r d342de2e09a4 relpipe-data/examples.xml
--- a/relpipe-data/examples.xml	Tue Dec 04 18:43:01 2018 +0100
+++ b/relpipe-data/examples.xml	Tue Dec 04 22:33:36 2018 +0100
@@ -38,6 +38,77 @@
 			If an error is found, it is reported on STDERR. So just omit the <code>&amp;</code> in order to see the error message.
 		</p>
 		
+		
+		<h3>/etc/fstab formatting using -in-fstab, -out-nullbyte, xargs and Perl</h3>
+		
+		<p>
+			As we have seen before, we can convert <code>/etc/fstab</code> (or <code>mtab</code>)
+			to e.g. an XML or a nice and colorful table using <m:name/>.
+			But we can also convert these data back to the <code>fstab</code> format. And do it with proper indentation/padding.
+			Fstab has a simple format where values are separated by one or more whitespace characters.
+			But without proper indentation, these files look a bit obfuscated and hard to read (however, they are valid).
+		</p>
+		
+		<m:pre jazyk="text" src="examples/relpipe-out-fstab.txt"/>
+		
+		<p>
+			So let's build a pipeline that reformats the <code>fstab</code> and makes it more readable.
+		</p>
+			
+		<m:pre jazyk="bash">relpipe-in-fstab | relpipe-out-fstab &gt; reformatted-fstab.txt</m:pre>
+			
+		<p>
+			We can hack together a script called <code>relpipe-out-fstab</code> that accepts relational data and produces <code>fstab</code> data.
+			Later this will be probably implemented as a regular tool, but for now, it is just an example of a ad-hoc shell script:
+		</p>
+		
+		<m:pre jazyk="bash" src="examples/relpipe-out-fstab.sh" odkaz="ano"/>
+		
+		<p>
+			In the first part, we prepend a single record (<code>relpipe-in-cli</code>) before the data coming from STDIN (<code>cat</code>).
+			Then, we use <code>relpipe-out-nullbyte</code> to convert relational data to values separated by a null-byte.
+			This command processes only attribute values (relation and attribute names are skipped).
+			Then we used <code>xargs</code> to read the null-separated values and execute a Perl command for each record (pass to it a same number of arguments, as we have attributes: <code>--max-args=7</code>).
+			Perl does the actual formatting: adds padding and does some little tunning (merges two attributes and replaces empty values with <em>none</em>).
+		</p>
+		
+		<p>This is formatted version of the <code>fstab</code> above:</p>
+		
+		<m:pre jazyk="text" src="examples/relpipe-out-fstab.formatted.txt"/>
+		
+		<p>
+			And using following command we can verify, that the files differ only in comments and whitespace:
+		</p>
+		
+		<pre>relpipe-in-fstab | relpipe-out-fstab | diff -w /etc/fstab -</pre>
+		
+		<p>
+			Regular implementation of <code>relpipe-out-fstab</code> will probably keep the comments
+			(it needs also one more attribute and small change in <code>relpipe-in-fstab</code>).
+		</p>
+		
+		<p>
+			For just mere <code>fstab</code> reformatting, this approach is a bit overengineering.
+			Wo could skip the whole relational thing and do just something like this:
+		</p>
+		
+		<m:pre jazyk="bash">cat /etc/fstab | grep -v '^#' | sed -E 's/\s+/\n/g' | tr \\n \\0 | xargs -0 -n7 ...</m:pre>
+		
+		<p>
+			plus prepend the comment (or do everything in Perl).
+			But this example is intended as a demostration, how we can
+			1) prepend some additional data before the data from STDIN
+			2) use <m:name/> and traditional tools like <code>xargs</code> or <code>perl</code> together.
+			And BTW we have implemented a (simple but working) <em>relpipe output filter</em> – and did it without any serious programming, just put some existing commands together :-)
+		</p>
+		
+		<blockquote>
+			<p>
+				There is more Unix-nature in one line of shell script than there is in ten thousand lines of C.
+				<m:podČarou>see <a href="http://www.catb.org/~esr/writings/unix-koans/ten-thousand.html">Master Foo and the Ten Thousand Lines</a></m:podČarou>
+			</p>
+		</blockquote>
+		
 	</text>
 
 </stránka>
diff -r 029dc972f1f2 -r d342de2e09a4 relpipe-data/examples/relpipe-out-fstab.formatted.txt
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/relpipe-data/examples/relpipe-out-fstab.formatted.txt	Tue Dec 04 22:33:36 2018 +0100
@@ -0,0 +1,6 @@
+# <file system>                                    <mount point>                            <type>           <options>                                          <dump>   <pass>
+UUID=29758270-fd25-4a6c-a7bb-9a18302816af          /                                        ext4             relatime,user_xattr,errors=remount-ro              0        1
+/dev/sr0                                           /media/cdrom0                            udf,iso9660      user,noauto                                        0        0
+/dev/sde                                           /mnt/data                                ext4             relatime,user_xattr,errors=remount-ro              0        2
+UUID=a2b5f230-a795-4f6f-a39b-9b57686c86d5          /home                                    btrfs            relatime                                           0        2
+/dev/mapper/sdf_crypt                              /mnt/private                             xfs              relatime                                           0        2
diff -r 029dc972f1f2 -r d342de2e09a4 relpipe-data/examples/relpipe-out-fstab.sh
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/relpipe-data/examples/relpipe-out-fstab.sh	Tue Dec 04 22:33:36 2018 +0100
@@ -0,0 +1,22 @@
+#!/bin/bash
+
+(
+	# Just troll-in the first record:
+	relpipe-in-cli generate Heathers 7 \
+		Duke string McNamara string Chandler string Veronica string J.D. string \
+		Big string Fun string \
+		'' '# <file system>' '<mount point>' '<type>' '<options>' '<dump>' '<pass>';
+		# relpipe-out-nullbyte processes only attribute values
+	# Read the actual pipe's input:
+	cat
+)	| relpipe-out-nullbyte \
+	| xargs --null --max-args=7 perl -e \
+		'printf("%-*s %-*s %-*s %-*s %-*s %s\n",
+			# following numbers define paddings:
+			50, ($ARGV[0] eq "" ? $ARGV[1] : "$ARGV[0]=$ARGV[1]"),
+			40, ($ARGV[2] eq "" ? "none" : $ARGV[2]),
+			16, $ARGV[3],
+			50, $ARGV[4],
+			 8, $ARGV[5],
+			$ARGV[6]
+		);'
diff -r 029dc972f1f2 -r d342de2e09a4 relpipe-data/examples/relpipe-out-fstab.txt
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/relpipe-data/examples/relpipe-out-fstab.txt	Tue Dec 04 22:33:36 2018 +0100
@@ -0,0 +1,13 @@
+# /etc/fstab: static file system information.
+#
+# Use 'blkid' to print the universally unique identifier for a
+# device; this may be used with UUID= as a more robust way to name devices
+# that works even if disks are added and removed. See fstab(5).
+#
+# <file system> <mount point>   <type>  <options>       <dump>  <pass>
+# / was on /dev/vda1 during installation
+UUID=29758270-fd25-4a6c-a7bb-9a18302816af /               ext4    relatime,user_xattr,errors=remount-ro 0       1
+/dev/sr0         /media/cdrom0   udf,iso9660 user,noauto  0       0
+/dev/sde /mnt/data ext4  relatime,user_xattr,errors=remount-ro 0       2
+UUID=a2b5f230-a795-4f6f-a39b-9b57686c86d5 /home  btrfs    relatime  0       2
+/dev/mapper/sdf_crypt /mnt/private xfs relatime  0       2
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